The tachometers generally used to measure the rotation speed of aircraft wheels are magnetic analogue tachometers (also called inductive analogue tachometers).
Such a tachometer conventionally includes a rotating portion securely fastened to the wheel and equipped with magnets or toothed elements made from a ferromagnetic material, and a fixed portion comprising a magnetic sensor (coil of conductive wire, Hall-effect sensor, etc.) delivering a periodic measurement signal generated by a magnetic-field variation resulting from the rotation of the wheel.
The frequency and/or amplitude of this measurement signal are proportional to the rotation speed of the wheel.
The measurement signal is acquired by an electronic measurement module connected by a cable to the tachometer.
When the frequency of the measurement signal is used to obtain the rotation speed of the wheel, the electronic measurement module estimates the time between two overshoots of a threshold (or current) voltage by the measurement signal, and deduces therefrom the rotation speed of the wheel.
Alternatively, the electronic measurement module uses a first voltage threshold for positive values of the measurement signal and a second voltage threshold for negative values of the measurement signal (hysteresis thresholding).
Such a measurement signal 1 is shown in FIG. 1. The electronic measurement module generates from the measurement signal 1 and from comparison with a first voltage threshold S1 and with a second voltage threshold S2 a logic signal 2. The logic signal 2 activates a “timer” on the rising state or falling state of the logic signal 2, thereby allowing a measurement of the period T and therefore of the frequency of the measurement signal 1 to be obtained. Thus a measurement of the rotation speed of the wheel is obtained.
This type of measurement has certain drawbacks. As the amplitude of the measurement signal is lower at lower rotation speed, there is, for a given voltage threshold, a frequency range (and therefore rotation-speed range) for which the frequency measurement is no longer possible. It would therefore seem advantageous to decrease this threshold voltage as much as possible. However, decreasing the threshold voltage makes the measurement much more sensitive to parasitic noise resulting from various perturbations undergone by the tachometer, the electronic module or the cable: electrical perturbations (noise of the components), electromagnetic perturbations (motors and other electrical equipment) and mechanical perturbations (repercussion of the vibrations and shocks undergone by the wheel during the measurement carried out by the tachometer).
Thus, if a threshold of 0 volts were used, even the slightest parasitic noise present in the measurement signal would corrupt the measurement of the period of the measurement signal.
FIG. 2 shows the impact on a measurement signal 3 of a white noise 4 caused by such perturbations, the measurement signal 3 being compared with a first threshold voltage S3 of 0.02 volts and a second threshold voltage S4 of −0.02 volts. The measurement of the period of the measurement signal 3 is perturbed by the presence of the white noise 4 once the amplitude of the white noise exceeds one of the first or second voltage thresholds S3, S4.
The voltage thresholds are therefore particularly complex to define since they affect not only the low-frequency measurement performance of the tachometer, but also the robustness to measurement noise.
It has thus been envisaged to filter the measurement signal so as to decrease the high-frequency perturbations located outside of the useful frequency band of the measurement signal. It is however impossible to filter the low-frequency noise resulting from perturbations of mechanical origin (for example generated by mechanical resonances at a few tens or hundreds of hertz), because their frequencies correspond to the useful frequency band of the measurement signal.
FIG. 3 shows a measurement signal 5 perturbed by a low-frequency perturbation. The measured period T′ of the logic signal 6 does not correspond to the rotation speed of the wheel. In this example, the wheel turns with the period 2T′, and the presence of perturbations of mechanical origin generates spectral lines of amplitudes higher than the amplitude of the useful spectral line (the frequency of which is the fundamental). This effect may be observed in the spectral representation of the signal in FIG. 6, in which the useful spectral line 16 has an amplitude lower than those of the harmonic spectral lines 17: in this case, the speed measurement is erroneous.